How The Shape Of Your Pelvis Can Affect How You Give Birth

Have you ever seen a child born to a woman that made you think, there's no way a baby that big came out of a woman that small? Cephalopelvic disproportion (CPD) is a term that is used to describe a condition in which a baby is too large to come through the mother's pelvis. The size and shape of a woman's pelvis vary. The position of baby often depends on the shape of mom's pelvis, such as how wide or how deep it is.

CPD is most often diagnosed in the second stage of labor (the pushing stage) when baby's head fails to descend through the pelvis. It is sometimes also referred to as "failure to descend" or "arrest of descent." In many cases, it happens due to baby's position, such as an occiput posterior position (face up), and baby's body doesn't rotate as it descends lower into mom's pelvis.1 It is possible, though, to deliver a baby in the occiput posterior position if mom's pelvis is large enough.

It may also occur if baby is not properly aligned (ascynclitic), leaning to the right or left.1

What Should Your OB Evaluate In The Prenatal period?

The size and shape of the pelvis matters

In some cases, CPD may occur if the dimensions of mom's pelvis are too narrow. There are four different types of pelvises:

  • Gynecoid, which is best suited for childbirth based on its dimensions
  • Android, where the pelvis is shaped a bit like a heart
  • Platypelloid, where the pelvis is broad and flat
  • Anthropoid, where the inlet is oval shaped

A woman may have one of these types, or a mixture of the four.2

Measuring the pelvis

Pelvimetry refers to specific measurements in areas of a woman's pelvis to determine whether or not the pelvis is large enough for the woman to deliver a baby vaginally. Pelvimetry may be performed during a vaginal exam. An X-ray of the pelvis can also be performed, but it provides very little information as to whether or not a baby is too large to fit through mom's pelvis.1

A woman's pelvis is divided into two sections, the greater (or true) pelvis and the lesser (or false) pelvis. It is the true pelvis that is important in childbearing.

Pelvis-400x208

There are three dimensions or measurements of the true pelvis. The most important measurements of the true pelvis involve the pelvic inlet, which is the smallest part that the baby's head must pass through.

Common Causes Of CPD

For some women, a small pelvis is simply genetic, and there are congenital conditions that could be a problem; you'll want to discuss those with your doctor during the early stages. There are, however, some common factors that can lead to CPD:

  • The mother being under 5 feet, 3 inches tall
  • History of tumors or fibroid
  • Gestational diabetes
  • Being over the age of 35
  • History of trauma to the pelvis or tailbone
  • Problems with bone density

Potential Risks When A Mother Has A Small Pelvis

Women with smaller pelvis have healthy babies every day – but if the size discrepancy is too great, the mother may need to undergo a C-section. If any of the measurements are smaller than normal, the pelvis may be contracted, which may make a vaginal delivery more difficult.2 This is because a smaller pelvis can stall the delivery, which can lead to serious birth injuries. Babies are at risk of developing:

  • Shoulder dystocia
  • Brachial plexus injuries
  • Cerebral palsy
  • Hypoxic ischemic encephalopathy
  • Intracranial hemorrhage
  • Umbilical cord prolapse

Prolonged labor is often treated with Pitocin. However, when the pelvis is too small, the baby can suffer injuries during labor. If CPD is not diagnosed, the mother is also at risk. A delayed C-section can cause problems for both the mother and the child. Your doctor should warn you about these risks before labor begins, so that you, your partner and your health care provider can create a plan for moving forward.

  1. Cunningham, F., Leveno, K., Bloom, S., Spong, C., Dashe, J., Hoffman, B., Casey, B., Sheffield, J. (2013). Abnormal Labor. In Cunningham, F., Leveno, K., Bloom, S., Spong, C., Dashe, J., Hoffman, B., Casey, B., Sheffield, J. (Eds), Williams obstetrics (24th ed.). New York, NY: McGraw-Hill
  2. Posner, G., Dy, J., Black, A., & Hones, G. (2013). Oxorn-Foote human labor and birth (6thed.). China: McGraw-Hill